Condensate water derived from the condenser of a steam turbine or turbines typically may contain impurities in the form of both (a) suspended and colloidally dispersed particulate solids and (b) dissolved ionic and nonionic solutes, including silica and silicates. The solute materials may be only intermittently present. The particulate solids are mainly iron oxide-containing corrosion products commonly called "crud". The condensate needs to be purified or "polished" before being recycled for reconversion to steam to avoid contaminant desposition upon such surfaces as turbine blades, boilers, pipes, and the like. In the case of condensate derived from the steam produced in a boiling water reactor (BWR), such contaminants may include radioactive substances, such as iron, or the like.
At BWRs constructed in recent years, condensate polishing has commonly been achieved by directing the condensate under pressure through vessels containing filter elements precoated with a thin layer of powdered ion exchange resin. Periodically, the precoat layer must be removed from the precoat filter elements and replaced with a fresh precoat layer. The material of the spent precoat layer is typically discarded. Examples of such precoat filter systems are disclosed, for example, in U.S. Pat. Nos. 3,250,703 and 4,269,707. In certain instances, the spent ion exchange resin from a discarded precoat layer is an undesirable source of a radioactive waste material, and the disposal thereof may create problems. A precoat filter is characteristically able to remove crud solids in a condensate down to levels that are about 2 to 3 ppb (parts per billion).
Recently, in Japan, a condensate polishing system has been proposed that utilizes a hollow fiber filter (HFF) in combination with a downstream mixed ion exchange resin bed demineralizer. An HFF utilizes a plurality of hollow fiber membranes that each have a pore size smaller than the size of suspended solids present in the condensate. The hollow fiber membranes are formed in a bundle and secured in place in a module that is supported vertically in a filter vessel. HFF filters are available commercially and methods for their manufacture are known to the prior art. When the condensate to be recycled is directed against the HFF, the suspended solid particles contained therein are retained on the fiber surfaces and the treated condensate flows through the hollow fiber membranes.
An HFF has been shown to be capable of reducing dispersed crud solids in a condensate down to levels that are as low as about 0.1 ppb. However, unlike a precoat filter, an HFF does not remove dissolved solute impurities. Unfortunately, dissolved impurities may at times be present in a condensate at levels which are undesirable, and even damaging, particularly when there is a condenser leak.
The current HFF containing-systems in use in Japan include deep bed demineralizers downstream of the HFF. The resulting combined system has sufficient capacity to remove dispersed crud solids, and ionic impurities and silica from an influent condensate undergoing purification for recycling purposes, so that such an HFF-containing system may be substituted for the conventional condensate polishing precoat filter.
However, with the exception of a few plants, the BWR plants in the United States that are equipped with precoat filter/demineralizer condensate polishing systems are typically not equipped with such deep demineralizer beds. Accordingly, the retrofitting of most existing BWR power plants in the United States to utilize HFF-containing systems based on such Japanese technology would require a substantial additional new capital investment to provide a deep bed demineralizer system to remove the ionic impurities and/or silica that may at times be present at objectionable levels in the condensate effluent from an HFF.
There is a need in the condensate polishing art for a system that permits economical utilization of hollow fiber or other type of membrane filters in existing United States BWR power plants. The resulting membrane-containing system needs to accomplish both crud removal and ion removal when ionic impurities (including silica) are intermittently present. Ideally, such a new system should be retrofittable to the existing precoat filter systems present in existing BWR power plants at relatively low cost and in a relatively simple manner. The present invention provides such a system.